In today's society, various materials such as paper, plastic and aluminum foil are used in the applications such as agricultural material, building material and packaging material for a variety of foods, drugs, sundry goods and the like taking a form of a liquid, a powder/granule or a solid. Above all, the plastic is excellent in the strength, water resistance, moldability, transparency, cost and the like and is being used as a molded article such as bag and container in a wide range of applications. The plastic that is at present widely used in the applications such as bag and container includes, for example, polyethylene, polypropylene, polystyrene, polyvinyl chloride and polyethylene terephthalate. However, the molded article composed of the plastic is not biodegraded or hydrolyzed in a natural environment or is degraded at an extremely slow speed and therefore, remains in the soil when buried after use or spoils the sight when dumped. Even when incinerated, there is a problem such as emission of a harmful gas or damage to the incinerator.
A biodegradable resin is attracting attention as an environment-friendly plastic capable of solving these problems. A film obtained by molding a biodegradable resin is degraded in the soil when buried in the soil after use and therefore, can contribute to preventing global warming or contamination of soil or air. For this reason, a film made of a biodegradable resin is often used for garbage bag, shopping bag and the like. However, many of these biodegradable resin-made films are generally poor in the mechanical properties, and in order to improve the mechanical properties of the biodegradable film while maintaining good biodegradability, a number of studies are being made.
For example, Patent Document 1 discloses a biodegradable resin composition obtained by mixing a starch, an aliphatic polyester and an aliphatic-aromatic polyester in a specific ratio. Also, Patent Document 2 discloses a biodegradable resin composition having a starch-containing dispersion phase and a continuous phase composed of a thermoplastic polymer. Furthermore, Patent Document 3 discloses a resin composition obtained by mixing a starch, an aliphatic polyester and a polyhydric alcohol in a specific ratio.
However, these biodegradable resin compositions disclosed in Patent Documents 1 to 3 suffer from poor dispersibility of the components in the resin composition, and improvement of mechanical properties is still insufficient. A biodegradable resin composition more improved in the mechanical properties and a molded body thereof are being demanded.
The aliphatic polyester resin is applied to various molded articles such as film, but when a molded article using the aliphatic polyester resin is left standing for a while, an oligomer, particularly a cyclic dimer, precipitates on the surface and disadvantageously whitens the surface, and an effective removal method therefor capable of keeping down the cost has not been developed so far. The method recently developed as an effective removal method includes a technique of reducing an oligomer, particularly a cyclic dimer, contained in the aliphatic polyester resin, and there is a known technique where the aliphatic polyester is washed with one or more kinds of solvents selected from an aliphatic ketone, a cyclic aliphatic ether and an aliphatic monoester at a temperature lower than the melting point of the aliphatic polyester and lower than the boiling point of the solvent (Patent Document 4). However, this method has a problem that the process of production steps is cumbersome and the organic solvent used remains in the resin.
A composition comprising a starch and an aliphatic polyester is prepared with an attempt to improve the mechanical property of the aliphatic polyester resin, but the above-described problem of whitening of the molded body due to an oligomer derived from the aliphatic polyester resin is not yet solved, and improvement of the mechanical property is also insufficient (Patent Document 5). In Patent Document 1, for the purpose of improving the mechanical property, a resin composition containing at least (A) an aliphatic polyester resin, (B) a thermoplastic resin other than the aliphatic polyester resin (A), (C) a starch and (D) a hydroxyl group-containing organic compound is disclosed.